A prior art switching element such as a high power bipolar transistor conducts a switching operation in response to a signal from a driver circuit.
FIGS. 4 and FIG. 5 show waveforms of input and output signals in a prior art driver circuit and in an ideal driver circuit, respectively.
As apparent from the comparison between the waveforms shown in FIG. 4 and 5, the delay times t.sub.1, t.sub.2 in between the response of the output signal (refer to FIG. 4 (b)) against the input signal (refer to FIG. 4 (a)) in the prior art driver circuit are both longer than those of the ideal case (t.sub.1 &gt;t.sub.3, t.sub.2 &gt;t.sub.4), and the rate of the rise time of the output signal di.sub.B1 /dt and the rate of the fall time of the output signal di.sub.B2 /dt are both smaller than those of the ideal case ((di.sub.B1 /dt)&lt;(di.sub.B3 /dt), (di.sub.B2 /dt)&lt;(di.sub.B4 /dt)). Furthermore, the time for converting a bias current into a reverse bias current is undesirably long.
Another prior art driver circuit is disclosed in an article entitled "SWITCHING PHENOMENA AND BASE DRIVE DESIGN" by TINUS VAN DE WOUW, PHILIPS SEMICONDUCTORS, PCI PROCEEDINGS, 1981. In this article it is disclosed "for a fast turn-on..., the base current should rise faster than the collector current," "A Darlington has a special property that the input transistor infact desaturates the output transistor...," and "if the input transistor is overdriven too much, a tail in the turn-off appears... . This tail is caused by the driver."